The present invention relates to a maximum likelihood sequence estimation receiver that estimates a transmission signal from a signal with transmission distortion due to a frequency-selective fading in multiple radio propagation (multipath) such as a radio line in a high-speed digital communications system, for example, a digital mobile telephone system. More particularly, the present invention relates to a maximum likelihood sequence estimation receiver that obtains estimates of transmission signals by selecting an optimum portion among impulse response sequences with transmission distortion. Moreover, the present invention relates to a method of receiving maximum-likelihood sequence estimates.
An example of the technique in which that type of signal estimation is performed by selecting the optimum portion among impulse response sequences with transmission distortion is disclosed in, for example, JP-A-292139/1993 titled as "maximum likelihood sequence estimation receiver".
FIG. 4 is a block diagram illustrating the configuration of a conventional maximum likelihood sequence estimation receiver. In FIG. 4, each of tap coefficients of the matched filter 22 is added based on impulse responses of a received input signal from the transmission line. In this case, the number of taps of the matched filter 22 has to be minimized to reduce the throughput of the status estimator 23 with the largest arithmetic amount. A reduced number of taps allows only a limited region of impulse response sequences to be processed.
Hence, it is needed to judge whether or not what region of impulse response sequences to be processed with a tap coefficient provides the highest estimation capability. The receiver shown in FIG. 4 implements an estimation region judgment according to the following steps. FIG. 5 is a diagram explaining impulse response values.
Referring to FIGS. 4 and 5, upon receiving a training signal from the transmission side, the signal generator 26 generates the same signal as the training signal. The estimator 25 determines characteristic-line impulse response values. When impulse response values are obtained as shown in FIG. 5, the position estimator 27 compares the amplitudes of the impulse response values. This comparison operation numbers the impulse response values in decreasing order. The region where the sum of pulse numbers is smallest among regions containing a pulse with the maximum amplitude is regarded as an optimum signal estimation region. The timing signal representing the optimum signal estimation region is sent to the matched filter 22 and the status estimator 23 to implement the optimum maximum likelihood sequence estimation.
However, the above-mentioned prior art requires to implement a complicated algorithmic operation to determine the optimum estimation region. That is, amplitude values, as shown in FIG. 5, need to be numbered in decreasing order. Moreover, the comparison operation must be repeated several times to determine the optimum estimation region by performing arithmetic to compare impulse response values. This results in making the whole algorithm complicated. Particularly, as the number of taps of the matched filter increases, the arithmetic operations to be processed increases.
The optimum estimation region judgment cannot always obtain an optimum estimation region because impulse responses necessarily contain a maximum value pulse.
FIG. 6 is a diagram explaining the status where impulses response values are numbered in a decreasing order of amplitude value in a transmission line. In order to estimate a signal with distortion in a transmission line with impulse responses as shown in FIG. 6, the pulses 2, 3, and 4 must be selected as optimum estimation regions. In this case, since the pulse 1 with the maximum value is necessarily selected, the optimum estimation region cannot be specified.